The U.S. Department of Justice sent subpoenas to the two largest issuers of securities based on subprime auto loans, leaving other subprime lenders to wonder what the agency is looking for and who might be subpoenaed next.

The Justice Department is looking into the underwriting and securitization of subprime auto loans. Securitization is the financial practice in which a lender sells off a bundle of loans to investors. In effect, the lender borrows money from the investors with which it can make new loans. The investors then get repaid, with interest, as the loans get repaid.

In filings with the Securities and Exchange Commission, GM Financial, of Fort Worth, Texas, and Santander Consumer USA, of Dallas, said they had received the subpoenas. Both companies said the Justice Department was seeking documents relating to the underwriting and securitization of subprime auto loans "since 2007."

The Justice Department has refused to comment.

"There are no allegations set forth in the subpoena," GM Financial said.

The Justice Department appears to be focusing on subprime auto lenders "with regard to representations made in the context of securitizations," Ken Rojc, managing partner in charge of the auto finance group for Chicago law firm Nisen & Elliott, said. "This doesn't appear to involve dealer rate participation or discriminatory lending practices."

The CEO of one subprime auto lender, who asked not to be named for fear of provoking regulators, said "I think it is just a massive, massive fact-finding mission to see if there's correlations between this and the mortgage sector."

But the differences between subprime auto lending and subprime mortgages, he added, were "like Earth and Mars."

He said, "I think some of this is driven by headlines in The New York Times and others." A July 27 Page 1 story in the newspaper compared recent growth in subprime auto loans and the subprime mortgage bubble that led to the Great Recession.

In the first half of 2014, the top three issuers of subprime auto asset-backed securities -- Santander, at $4.75 billion; GM Financial, at $2.15 billion; and Exeter Finance Corp., at $1.15 billion -- accounted for 70 percent of dollar volume for the whole market, according to Standard & Poor's Ratings Services.

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